It's got a fantastic concept, title and cast, and some very funny moments and nice scenes, but it's not a complete slam dunk. First of all Jason Segel, who you would think would be perfect (and is certainly perfectly cast, and co-wrote the script) isn't in good form - he seems distracted, and not all quite there; he's going through the motions, and relying on his technique but his game isn't "on" in the way that Cameron Diaz's definitely is. She throws herself into her part and his obviously keen on aging disgracefully, showing off her backside and generally being raunchy. They should be a perfect couple but Segel's unfocused performance stops the fun.
Secondly, there's the plotting; this is set up as a farce, but there are too many holes in the storyline - the stakes are set up being about not wanting family-image-friendly Rob Lowe finding out about the sex tape, but a third of the way in he's revealed to be into cocaine (a really funny sequence) so the stakes go away; they try to reactivate it with a blackmailing kid but it doesn't hold. There's also too much of characters "oh I didn't realise you could do that" or "that doesn't matter".
I love Ellie Kamper but her part seems a bit of a waste - she and Rob Coddrry are Diaz and Segel's best friends, and you keep expecting them to complicate the action somehow, but they don't, really Characters come in who you think are going to do something interesting - eg Diaz's mother, Segel's co worker - but they disappear. The movie needed a genuine villain or something else - some structural work.
But there are lots of laugh out loud moments and the movie does say something about marriage. It is also better directed than Jake Kasdan's previous feature, Bad Teacher.